The present invention relates to a spectacle lens for adjusting eye sight. In particular, the present invention relates to a spectacle lens used for correction of presbyopia.
Conventionally, spectacle lenses for correction of presbyopia such as a unifocal lens, a multifocal lens and a progressive power lens are widely used to compensate for age-related decrease in amplitude of accommodation of eye. Since these spectacle lenses have advantages and drawbacks in comparison with each other, an wearer of the spectacle lenses for correction of presbyopia is required to choose one of the spectacle lenses according to conditions and environments so as to obtain eyesight adapted to object distance that the wearer mainly uses.
As personal computers (PCs) become widespread, operating time of the PCs by elderly persons is becoming greater every year. For this reason, a desire for comfortably viewing an object such as PC's display at a middle distance of about 50 cm is growing.
The unifocal lens refractive power of which is adjusted to the middle distance fits the desire because the unifocal lens is suitable for use at middle distances. However, due to its characteristics, the unifocal lens can be applied to only a shallow range in which accommodation of eyes acts. Therefore, the wearer is required to change the spectacles when the wearer views an object at a long distance or views an object at a short distance. This is very troublesome for the wearer.
Computer operation requires an operator to simultaneously view an object such as a document at the short distance and an object such as a computer screen at the middle distance. Under such a situation, usefulness of the unifocal lens lowers.
Although the multifocal lense can be used for the computer operation (i.e., for simultaneously viewing the object at the short distance and the object at the middle distance), the multifocal lens causes the wearer a sense of discomfort due to jumps of images resulting from discontinuity of the refractive power of the multifocal lens. Furthermore, in a case of the multifocal lens having high addition power, the wearer (wearer's eye) may fail to focus on objects at the middle distance since such multifocal lens is designed to have a relatively great difference in refractive power between a distance portion for distance vision and a near portion for near vision.
To solve the above mentioned problems of the unifocal lens and the multifocal lens, some types of progressive power lenses which are designed to simultaneously view objects at the middle distance and at the short distance have been suggested.
U.S. Pat. No. 4,786,160 (document 1) discloses a spectacle lens which has progressive addition power in its upper portion and lower portion. The spectacle lens of the document 1 further has a central portion which has substantially constant refractive power and which is located between a distance portion and a near portion.
To provide the spectacle lens with three visual distances (i.e., the distance portion for distance vision, the middle portion for middle vision and the near portion for near vision) enables the spectacle lens to be used for a full range of distance (i.e., from the short distance to the long distance). However, in this case, vertical size of each of the distance portion, the middle portion and the near portion becomes smaller. Furthermore, since the spectacle lens is provided with two progressive power portions in addition to the three visual distance portions, a change in refractive power in each progressive power portion becomes sharp. Because the spectacle lens in the document 1 has the same problems as those of the multifocal lens in addition to having a difficulty of machining process, usefulness of the spectacle lens of the document 1 is very low.
In all cases of progressive power lenses disclosed in U.S. Pat. No. 5,805,265 (document 2), U.S. Pat. No. 5,864,380 (document 3) and U.S. Pat. No. 5,777,716 (document 4), an object at middle distance is viewed through a progressive power portion located between a distance portion and a near portion. Since each of spectacle lenses disclosed in the documents 2–4 has great astigmatism within side portions of the progressive power portion (i.e., a central portion), there is a limitation in making a clear vision area wide in the central portion and therefore distortion occurs in the central portion.
In this specification, the clear vision area means an area in which an image can be viewed without distortion or blurring. More specifically, the clear vision area means an area in which astigmatism is less than or equal to 0.5D [Diopter]. Expressions of directions such as “upwards”, “downward”, “horizontal” and “vertical” indicate directions with reference to a condition during wearing of the spectacle lens.
In general, during the PC operation, a display and a keyboard respectively located at the middle distance and the short distance are alternately viewed at frequent intervals. In a case where the object at the middle distance is viewed through the central portion of the spectacle lens of the documents 2–4, refractive power changes as an angle of the wearer's line of sight changes even if the object distance does not change.
Therefore, the wearer is required to shake his/her head vertically or horizontally so as to find a condition in which an image formed through the progressive power portion is comfortably viewed. Such adjustment makes the wearer tired and therefore work efficiency lowers.